Method for enhancing antifouling properties of high entropy alloys

ABSTRACT

The invention relates to a method capable of enhancing both the antifouling properties of a high entropy alloy and also the mechanical properties thereof, without using chemical substances such as antifouling paints to prevent the attachment of animals and plants such as barnacles, mussels, sea lettuces, diatoms, etc., the method according to the invention being characterized by the use of severe plastic deformation to reduce the grain size in a high entropy alloy and thereby impart to the high entropy alloy a property of suppressing the attachment of aquatic or marine organisms, and according to the method, environmental issues caused by the use of antifouling paint, which is conventionally the most widely used method, may be resolved, and the energy efficiency of ships may be improved, thereby reducing societal and economic loss.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for enhancing the antifouling properties of high entropy alloys, and more particularly, to a method for enhancing the antifouling properties of high entropy alloys, wherein the attachment of animals and plants such as barnacles, mussels, sea lettuces, diatoms, etc., may be prevented without using chemical substances such as antifouling paints, while also improving the physical properties of the high entropy alloys.

2. Description of the Related Art

Marine organisms, such as various types of plankton, algae, shellfish, etc. attach to water contacting parts of marine structures. When the amount of organisms attached to a marine structure or ship increases, there is an increase in the weight of the marine structure or ship and an increase in the surface area of portions thereof which are exposed to water pressure due to ocean waves, and thus the functionality of the marine structure or ship is greatly reduced.

For example, when the thickness of a fouling layer is 0.2 cm, the velocity of a ship is reduced by 20% and the fuel consumption increases by up to 40%. Moreover, the holes of fishing nets are blocked, inhibiting the supply of oxygen and thereby causing the fish in the fishing nets to die off, which results in significant losses to the fishing industry. In addition, cleaning and replacement of damaged structures results in dramatic societal and economic costs. Antifouling construction is performed to prevent such costs.

The following methods are used to prevent fouling. 1) White or bright colored paints are used to reduce the amount of attaching organisms. 2) Coating layers are made dense and even to reduce the amount of attaching organisms. 3) Poisonous substances are added to paints.

Among such methods, 1) and 2) are still being studied. In practice, although methods such as using antifoulants that contain poisonous substances such as tributyltin (TBT) or applying slippery silicone based paints to prevent the attachment of contaminating organic substances to ship bodies are being used. However, TBT usage has recently caused significant environmental problems, which has resulted in TBT being legally regulated around the world starting in 2000, and thus difficult to use, and silicone based paints are limited in that the effectiveness of the antifouling effect is poor. Therefore, various antifoulants are being developed to replace TBT, but do not fundamentally resolve the environmental problems.

High entropy alloys (HEA) are not only multi-element alloy systems of five or more elements, but also have high mixing entropies. Consequently, high entropy alloys are novel materials which have excellent ductility and do not form intermetallic compounds, and are composed of a single face centered cubic (FCC) or body centered cubic (BCC) phase.

It was discovered in 2004 that when similar fractions of five or more elements are alloyed without there being a main element, a single phase material excluding intermediate phases is obtained, and such materials were presented to the academic community as high entropy alloys (HEA). A recent dramatic increase in interest has led to an explosion in research related to high entropy alloys.

Although the reason behind the formation and properties of such particular atomic arrangement structures is not yet clear, the superb chemical and mechanical properties exhibited by the structures have been reported. Face centered cubic (FCC) single phase Co—Cr—Fe—Mn—Ni based high entropy alloys form nanoscale twins at low temperatures, and thus have high yield and tensile strengths, and have the highest reported toughness to date.

That is, at extremely low temperatures, high entropy alloys having face centered cubic (FCC) structures not only have excellent fracture toughness, but also have excellent corrosion resistance and superb mechanical properties such as high strength and ductility, and thus are highly likely to be used in marine structures or ships operated in extreme low temperature environments.

Therefore, improving the inherent antifouling properties of high entropy alloys could be considered a method for expanding the range of possible applications for high entropy alloys in marine structures or ships.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for enhancing the antifouling properties, that is, the ability to prevent the attachment of marine organisms such as barnacles, mussels, sea lettuces, and diatoms, of high entropy alloys used in ships or marine structures.

In order to address the above-identified problem, there is provided a method for enhancing the antifouling properties of high entropy alloys, the method being characterized by the use of severe plastic deformation to reduce the grain size in a high entropy alloy and thereby impart to the high entropy alloy a property of suppressing the attachment of aquatic or marine organisms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a microstructure observed using transmission electron microscopy after a high pressure torsion process is used to reduce the grain size in a high entropy alloy according to an embodiment of the invention;

FIG. 2 illustrates a process for evaluating the antifouling properties of a material according to an embodiment of the invention or a comparative embodiment; and

FIG. 3 illustrates the difference in tensile properties whether or not a high pressure torsion process was performed according to the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Hereinafter, detailed description is given of methods according to exemplary embodiments of the invention, with reference to the accompanying drawings. However, the invention is not limited to the embodiments described below. Therefore, it will be obvious to a person with ordinary skill in the art that various modifications could be made to the invention without departing from the technical scope thereof.

As a result of pursuing research aimed at enhancing the antifouling properties of high entropy alloys which are likely to be used in marine structures or ships subject to extremely low temperatures, the present inventors were able to develop the invention by discovering that when severe plastic deformation is used to reduce the grain size in a high entropy alloy, not only are the antifouling properties enhanced to a significant degree, but the mechanical properties may also be improved such that the range of possible applications for the high entropy alloys is expanded.

That is, the invention is characterized by the use of severe plastic deformation to reduce the grain size in a high entropy alloy and thereby impart to the high entropy alloy a property of suppressing the attachment of aquatic or marine organisms.

Although the composition of the high entropy alloy is not particularly limited, since, as described above, high entropy alloys having a single face centered cubic (FCC) phase are highly likely to be used in marine structures or ships having excellent low temperature properties, FCC single phase high entropy alloys, which include, for example, widely known Co—Cr—Fe—Mn—Ni based alloys, and other high entropy alloys having compositions capable of forming an FCC single phase may be used without limit.

Methods including a high pressure torsion process or an equal-channel angular pressing (ECAP) process may be used as the severe plastic deformation process.

Meanwhile, when using the high pressure torsion process, it is desirable to reduce the grain size in an object subject to deformation, by performing the torsion process at least 5 times, desirably at least 7 times, and more desirably at least 10 times.

EXAMPLE Example 1

In Example 1 of the invention, a Co—Cr—Fe—Mn—Ni based high entropy alloy was used, having excellent mechanical properties and excellent corrosion resistance, which, in particular, is an essential property for marine structures and ships.

The specific composition of the alloy is 10 at % of Co, 15 at % of Cr, 25 at % of Fe, 10 at % of Mn, and 40 at % of Ni.

First, a specimen for a high pressure torsion process was prepared using a high entropy alloy sheet (the grain size of the sheet is 15 μm) having a diameter of 10 mm and a thickness of 1.5 mm, and a torsion process in which the sheet was rotated under a pressure of 5 GPa was performed 10 times in order to reduce the structure of the material.

FIG. 1 is a photograph illustrating the microstructure of a high entropy alloy observed using transmission electron microscopy after a torsion process according to Example 1 of the invention is performed. As shown in FIG. 1, it was observed that the grains were reduced to have an extremely fine grain size of 100 nm or smaller. Here, grain size indicates the length of the grain in the direction of the minor axis.

Five specimens subjected to such a high pressure torsion process were prepared, and after polishing the specimens using, in order, 400, 600, 800, and 1200-grit sandpaper, and using a diamond suspension to perform a 1 μm polish and thereby obtain a mirror finish to minimize the effects of surface roughness on each of the specimens, the antifouling properties of the specimens were evaluated.

Example 2

In order to confirm whether high pressure torsion deformation according to the invention can be used to obtain the same effect in other metal materials, a sheet (the grain size of the sheet is 20 μm) formed of STS304 stainless steel was prepared under the same conditions as Example 1.

Comparative Example 1

For comparison with the above Examples 1 and 2, a glass sheet was prepared to have the same size as in Examples 1 and 2, and the antifouling properties were evaluated through the same polishing operation.

Comparative Example 2

For comparison with the above Example 1, the high entropy alloy sheet in Example 1 was formed into the same size as in Example 1, and then, without performing the high pressure torsion process, the antifouling properties were evaluated through the same polishing operation.

Comparative Example 3

For comparison with the above Example 2, the stainless steel sheet in Example 2 was formed into the same size as in Example 2, and then, without performing the high pressure torsion process, the antifouling properties were evaluated through the same polishing operation.

FIG. 2 illustrates a process for evaluating the antifouling properties of specimens according to Examples 1 and 2 of the invention and Comparative Examples 1 to 3.

Specimens prepared for antifouling property evaluation were stored in a growth chamber at 20° C., and in order to test the fouling of a single cell, a single cell suspension was obtained using a nylon mesh.

After immersing the metal specimens for 12 hours in the suspension, the metal specimens were reacted for two days in a biofilm reactor. Here, the spores used in the test are diatom and Porphyra suborbiculata spores.

Table 1 below shows the total amount of carbohydrates in diatoms, derived from the antifouling property evaluation tests.

TABLE 1 Total amount of carbohydrates Specimen (mM/mL/cm²) Comparative Example 1 3.8 ± 0.3 Comparative Example 2 2.6 ± 0.6 Comparative Example 3 2.4 ± 0.5 Example 1 2.1 ± 0.2 Example 2 1.8 ± 0.3

As seen in Table 1 above, both the conventional high entropy alloy with coarse grains (Comparative Example 2) and the stainless steel (Comparative Example 3) exhibited better antifouling properties with respect to diatoms than the glass sheet (Comparative Example 1) used as the control group.

That is, with respect to diatoms, the antifouling properties of the metals may be said to be superior to those of the glass sheet.

Meanwhile, when the antifouling properties of the high entropy alloy were enhanced by reducing the grain size in the high entropy alloy, the fouling with respect to diatoms was reduced by 19%, and a fouling reduction effect of 25% could be observed for the stainless steel.

That is, it is seen that the antifouling properties of high entropy alloys with respect to diatoms may be enhanced to a significant degree through a high pressure torsion process according to an embodiment of the invention.

Table 2 below shows the total amount of carbohydrates in Porphyra suborbiculata spores, obtained from the antifouling property evaluation tests.

TABLE 2 Total amount of carbohydrates Specimen (mM/mL/cm²) Comparative Example 1  1.6 ± 0.15 Comparative Example 2  3.1 ± 0.61 Comparative Example 3 3.06 ± 0.46 Example 1 1.30 ± 0.17 Example 2 2.79 ± 0.87

Table 2 above shows the fouling results using Porphyra suborbiculata spores, and show that both the prepared high entropy alloy with coarse grains (Comparative Example 2) and the stainless steel (Comparative Example 3) exhibited worse antifouling properties than the glass sheet (Comparative Example 1) used as the control group.

That is, with respect to Porphyra suborbiculata spores, the antifouling properties of metals such as the high entropy alloy or the stainless steel are observed to be inferior to those of the glass sheet

However, when the high torsion process was used to reduce the grain size, the high entropy alloy (Example 1) exhibited a 58% reduction in fouling and a fouling reduction effect of 9% was observed for the stainless steel (Example 2).

That is, it is seen that when the high pressure torsion process according to the invention is applied to a high entropy alloy, the antifouling properties with respect to Porphyra suborbiculata spores may be significantly enhanced.

FIG. 3 illustrates the change in strength through grain size reduction by high pressure torsion process. As seen in FIG. 3, it is seen that when the coarse grains in a high entropy alloy according to the invention are reduced in size through a high pressure torsion process, the tensile strength is enhanced from about 600 MPa to at least 1800 MPa.

High entropy alloys have lower strength than conventional steels, but when a high pressure torsion process according to an embodiment of the invention is performed, not only the strength may be significantly improved, but the antifouling properties may also be significantly enhanced.

According to the invention, a deformation technique called severe plastic deformation may be used to significantly reduce the grain size in a high entropy alloy and thereby enhance the antifouling properties of the high entropy alloy itself. Consequently, by resolving the environmental issues caused by applying conventionally used antifouling paints and improving the energy efficiency of ships, the societal and economic loss due to the fouling of ships and marine structures may be reduced.

Moreover, a reduction in grain size resulting from a severe plastic deformation technique according to the invention may, in addition, significantly improve the mechanical strength of a high entropy alloy, and thus the range of applications may also be broadened for high entropy alloys, which are lower in strength than conventional materials. 

What is claimed is:
 1. A method for enhancing the antifouling properties of a high entropy alloy, the method comprising: using severe plastic deformation to reduce the grain size in a high entropy alloy and thereby impart to the high entropy alloy a property of suppressing the attachment of aquatic or marine organisms.
 2. The method of claim 1, wherein the severe plastic deformation technique is a high pressure torsion process.
 3. The method of claim 1, wherein the high pressure torsion process is performed at least five turns.
 4. The method of claim 1, wherein the severe plastic deformation technique is an equal-channel angular pressing (ECAP) process.
 5. The method of claim 1, wherein the high entropy alloy has a face centered cubic (FCC) structure.
 6. The method of claim 1, wherein, in the high entropy alloy having the reduced grain size, the grain size is at most 100 nm. 